This study examined the effects of cognitive-behavioral therapy (CBT) for principal panic disorder with or without agoraphobia, on comorbidity in 30 individuals (16 female). To test the hypothesis that improvements in co-existing conditions were not due to spontaneous fluctuations across time, patients receiving immediate CBT were compared to those assigned to wait list (n = 11). Results indicated clinician-rated severity of comorbid specific phobia declined significantly following immediate CBT compared to no change after wait list. The number of patients without comorbidity of any severity increased after immediate CBT, with no such increase following wait list. However, the groups did not differ in the frequency of additional diagnoses or overall severity of comorbidity. In the total sample, results indicated reductions in comorbidity by 9-month follow-up, with marked declines in the severity of comorbid generalized anxiety disorder (GAD), social and specific phobia. Our findings suggest that targeted CBT for panic disorder has beneficial effects on comorbidity over the longer term and that some of its immediate effects exceed those due to the passage of time alone.